Linear and nonlinear effects distort optical signals transmitted over optical fibers. Such effects include chromatic dispersion (CD) and self-phase modulation (SPM). Optical dispersion compensation is typically employed to reduce signal distortion that arises as a result of CD.
Electronic dispersion compensation (EDC) has recently emerged as a technique that can flexibly reduce the distortion induced by CD in a cost effective manner. As explained by M. S. O'Sullivan, K. Roberts, and C. Bontu, in “Electronic dispersion compensation techniques for optical communication systems,” ECOC'05, paper Tu3.2.1, 2005, EDC can be performed at the transmitter. Doing so is referred to herein as pre-EDC. Alternatively, as described by S. Tsukamoto, K. Katoh, and K. Kikuchi, in “Unrepeated Transmission of 20-Gb/s Optical Quadrature Phase-Shift-Keying Signal Over 200-km Standard Single-Mode Fiber Based on Digital Processing of Homodyne-Detected Signal for Group-Velocity Dispersion Compensation,” IEEE Photonics Technology Letters, Volume 18, Issue 9, 1 May 2006, pp. 1016-1018, EDC can be performed at the receiver, which is referred to herein as post-EDC.
Post-EDC has an advantage over pre-EDC, in that post-EDC does not require that performance feedback be supplied from the receiver to the transmitter. Unfortunately, direct intensity detection, also known as square-law detection, which is the commonly used optical detection technique of today's optical fiber communications systems, e.g., the optical to electronic conversion performed by photodiodes, only recovers the optical signal amplitude and cannot recover the optical signal phase information, thus making the performance of post-EDC much poorer than that of pre-EDC.
To overcome this shortcoming, and hence enhance the performance of the post-EDC, the Tsukamoto et al. article suggests employing coherent detection to fully reconstruct the optical signal's complex field, i.e., both amplitude and phase. However, disadvantageously, as compared to direct intensity detection, coherent detection is much more sophisticated, and hence more expensive and difficult to perform. Further disadvantageously, coherent detection requires the use of an optical local oscillator (OLO), as well as phase and polarization tracking between the OLO and the signal carrier.